Data signals which have been transmitted from a transmitter can be received at a receiver. The data signal is transmitted at a carrier frequency and may contain digital data in the form of data symbols. The data signals may be wireless signals such as radio signals transmitted through a wireless channel, or wired signals transmitted on a wired channel, such as using a coaxial cable.
A receiver operates at a baseband frequency. In order to correctly receive a digital data signal, the data signal is downmixed from the carrier frequency to the baseband frequency. In order to do this, the receiver determines the carrier timing (i.e. the carrier frequency and phase) of the waveform of the received data signal. As well as determining the carrier frequency and phase, the timing of the data symbols is determined so that the receiver can sample the data symbols at the correct frequency and phase (i.e. at the correct timing) in order to faithfully reproduce the transmitted data symbols.
The receiver uses a receiver carrier timing which it can adjust to match (or “lock onto”) the carrier timing of the data signal. Similarly, the receiver uses a receiver symbol timing which it can adjust to match (or “lock onto”) the symbol timing of the data signal. In order to adjust the receiver timing (where the “receiver timing” is e.g. the receiver carrier timing and/or the receiver symbol timing), the receiver determines an indication of a phase error between the receiver timing and the signal timing (where the “signal timing” is e.g. the data signal carrier timing and/or the data signal symbol timing), and then adjusts the receiver timing based on the determined phase error indication to thereby reduce the error. This can be performed iteratively to bring the receiver timing closely in synchronisation with the signal timing. There are phase error algorithms which determine phase error indications (or “error metrics”) for carrier and symbol phase offsets by sampling the data signal in accordance with the receiver timing and measuring a phase error for individual samples. However, when such phase error algorithms (which measure errors for individual samples) are used in the presence of noise or channel distortion, the phase error indications that are determined may be unreliable due to the noise and/or channel distortion. To address this issue, the phase error indications can be averaged over an averaging period including many samples in order to achieve a more stable and accurate phase error indication. The average phase error indications can then be passed to Phase-Locked Loops (PLLs) controlling the receiver's local oscillators and sample timing hardware to match the receiver carrier and symbol timing to the signal carrier and symbol timing of the incoming data signal.
For the case of symbol timing, any error (referred to herein as “sample rate offset”) between the rate (i.e. frequency) at which the receiver samples the received signal and the rate at which the data symbols occur in the data signal will cause the sample points to slip (or “drift”) relative to ideal sample points. As the timing of the receiver sampling varies relative to the ideal sample timing (according to the signal symbol timing), the phase error will vary over the averaging period. Averaging the varying phase error indications over the averaging period will reduce the accuracy of the phase error measurement if the phase error has varied significantly over the averaging period. There is a limit to how much the phase error measurements can vary over the averaging period before the phase error measurements stop being useful for matching the receiver timing to the signal timing. If the offset in the sample timing used by the receiver relative to the signal symbol timing varies by more than a fraction of a symbol period over the averaging period, then the average phase error indication determined by the phase error algorithm may cease to be a reliable measure with which to correctly adjust the receiver symbol timing, and signal acquisition may fail at the receiver. A similar limit applies for adjusting the receiver carrier timing to match the signal carrier timing, where the phase error indications determined by a phase error algorithm may become meaningless if the phase error changes significantly over an averaging period.
Therefore, the phase error algorithms described above are used for performing fine timing synchronisation once a low symbol and/or carrier rate offset has been achieved. In order to achieve a low symbol and carrier rate offset, other methods are initially used to perform coarse timing synchronisation to achieve sufficiently low symbol and carrier rate offsets such that the phase error algorithms described above can be applied for the fine timing synchronisation. Therefore the receiver implements two phase error algorithms (one to perform coarse timing synchronisation and one to perform fine timing synchronisation) and implements a procedure to handover between the two algorithms.